The invention relates to a method and an apparatus for monitoring the thickness of a film. More specifically, the invention relates to a method and an apparatus for providing information representative of the thickness of an ink film in a printing press.
Many known devices provide information about the thickness of a film. In some known devices, electrical transducers are used. The transducers may be of the type which have electrical properties that vary in dependence upon the thickness of the film. The transducer may sense changes in inductance, capacitance or resistance caused by changes in the thickness of the film passing thereby.
U.S. Pat. No. 3,857,095 discloses an electromagnetic sensor whose inductance changes with changes in the thickness of an ink film on a roller which supplies ink to a printing roll. In U.S. Pat. No. 4,345,203, a capacitance transducer is used for measuring the thickness of a film of lubricant between two relatively rotatable surfaces. With these known devices, the measured electrical properties may vary significantly with variations in the electrical properties of the film. Therefore, the measured properties could vary significantly with variations in density, water content, temperature, etc. of a liquid film.
One of the ways to alleviate these problems is to use magnetic reluctance sensing. Under this method, a magnetic flux is established between a member that carries the film and a member that carries a magnetic transducer for sensing variations in the film thickness. Such a transducer generally includes a core and an excitation coil associated with the core to establish a magnetic flux across the film. A sensing means senses variations in the magnetic flux resulting from changes in the thickness of the film. Such devices are shown for example in U.S. Pat. No. 3,922,599.
A magnetic transducer for measuring the thickness of a lubricant film in a bearing is described in the French article "Measuring without A Contact", CETIM-Informations, No. 76. In this transducer, a U-shaped core is employed with core legs facing the film of lubricant. Excitation and sensing coils are mounted around the core.
It is known that the reluctance of a magnetic circuit is: ##EQU1## where: A is the area of the pole face,
M.sub.o is the free space permeability, PA1 M.sub.e is the permeability of the magnet alloy, PA1 L.sub.m is length of the magnetic core, and PA1 d is the distance or gap from the core to the liquid film carrying member. (The gap appears at each pole of a U-shaped core, hence the factor of 2.)
Generally, M.sub.e is significantly greater than L.sub.m, therefore, the first term in brackets is quite small and can be neglected.
The magnetic flux is determined from an equation: ##EQU2## where F is the magneto-motive force and is equal to N.sub.e I, where N.sub.e is number of turns of the excitation coil, and I is the current.
Substituting R for its expression, the magnetic flux will be: ##EQU3##
The voltage induced in the sensing coil is: ##EQU4## where N.sub.s is number of turns in the sensing coil.
Substituting dQ for its expression (Equation 3) the induced voltage is: ##EQU5## In known methods, d is generally kept constant, and generation of voltage occurs by changing the current, I.
Many prior art film thickness measuring devices use alternating current to energize an excitation coil of an electromagnet. However, the use of alternating current results in parasitic flux in the transducer. The presence of the parasitic flux in a transducer does not allow a number of transducers to be connected in series. Such a series connection is generally advantageous when the thickness of a liquid film has to be determined at predetermined positions across the width of the film.